Monoamine oxidase activity in the blood platelets of psychiatric patients is being studied prior to and during treatment with psychotropic drugs. We have found schizophrenic patients (Research Diagnostic Criteria) who hallucinate have significantly lower platelet MAO activity than schizophrenics who do not hallucinate, patients with affective psychoses and normal controls. No effect psychotropic drugs on platelet MAO activity was found. Platelet counts were found to be superior to platelet protein as a technique for determining the specific activity of platelet MAO activity. The proportion of heavy platelets with high MAO activity appears to be more variable in psychiatric patients than in normal controls. Serum DBH activity is normal in schizophrenics with a tendency for chronic paranoid schizophrenics to be significantly lower than those of other types of schizophrenics. We have also confirmed our finding of decreased serum DBH activity in unipolar psychotically depressed patients (14.7 plus or minus 8.7 micron mol/min/liter, N equals 18) compared to 20.1 plus 12.02 for normal controls (N equals 90) and bipolar depressed patients (21.4 plus or minus 9.4, N equals 13) for bipolar depressed patients. The range of variation in serum DBH activities in morning samples in patients and controls was not significantly different but is greater by far than previously believed. Evidence for DBH inhibitors was found in sera of some patients.